1. Field of the Invention
This invention relates to a method of forming microcapsule films in a polar liquid. In greater detail the present invention relates to a method of forming (modifying) microcapsule films using polyfunctional amines (including derivatives thereof). The microcapsule films obtained by the method of the present invention have low porosity, are not water permeable, have low light transmission, are difficulty swelled by water or moisture, and are thick and strong.
2. Description of the Prior Art
Hitherto, many methods for encapsulating hydrophilic materials in a polar liquid are known.
The microcapsules function to change the apparent state and property of the materials, to protect the materials in a very fine state, to control the discharging ability and to discharge the contents thereof at an appropriate time.
The functions of the microcapsules are as follows:
A. IT IS POSSIBLE TO CHANGE A LIQUID MATERIAL INTO AN APPARENTLY SOLID MATERIAL,
B. IT IS POSSIBLE TO MODIFY THE WEIGHT AND QUANTITY OF MATERIALS,
C. IT IS POSSIBLE TO CONTROL THE DISCHARGING OF THE MATERIALS CONTAINED IN THE CAPSULES,
D. IT IS POSSIBLE TO ISOLATE REACTIVE MATERIALS AND THUS TWO OR MORE REACTIVE MATERIALS CAN BE CONTAINED AT THE SAME TIME IN THE SAME SYSTEM FOR A LONG PERIOD OF TIME OR MATERIALS INCLUDED CAN BE PROTECTED FROM EXTERNAL INFLUENCES OR STORED FOR A DESIRED PERIOD OF TIME,
E. IT IS POSSIBLE TO SHIELD THE COLOR, THE FLAVOR AND THE VIRULENCE OF THE MATERIALS CONTAINED, AND
F. THEY HAVE THE PROPERTIES OF A FINELY DIVIDED POWDER.
Much research has been conducted to apply these functions to recording materials, medical supplies, perfumes, agricultural chemicals, chemicals, adhesives, liquid crystal paints, foods, detergents, dyestuffs, solvents, catalysts, enzymes and rust inhibitors, etc. Pressure sensitive copying sheets, aspirin capsules, perfume containing capsules, menthol containing capsules, pressure sensitive capsule adhesives, rust inhibitor containing capsules used for riveting, liquid crystal containing capsules and insecticide containing capsules have been practically used.
The methods of encapsulating can be classified into chemical processes, physico-chemical processes and physico-mechanical processes. Further, combinations of these processes can be utilized.
Methods of producing microcapsules are illustrated in the following in greater detail.
As the methods for microencapsulating utilizing chemical processes, an interfacial polymerization process and an "in situ" polymerization process are known.
Microencapsulation using the interfacial polymerization process utilizes a reaction for synthesizing polymers. The interfacial polymerization process is reported in Journal of Polymer Science, 60, 299 (1950).
In this process, an interfacial polymerization reaction is utilized using a combination of a hydrophobic monomer (or a prepolymer thereof) and a hydrophilic monomer (or a prepolymer thereof). Namely, the hydrophobic monomer is added to an organic medium which has no affinity to water, and the solution is finely dispersed in an aqueous phase. Then a water soluble or water dispersible monomer is added to the aqueous phase, by which the polymerization reaction occurs at the water and oil interfaces to form polymer films. Compounds used for such film formation are polyfunctional materials which cause a polycondensation reaction or an addition polymerization reaction. Thus, the formed capsules have a polyamide, polyester, polyurethane or polyurea film.
A number of patents concern encapsulation utilizing this principle, as disclosed, for example in Japanese Pat. Publication Nos. 19574/63, 446/67, 771/67, 2882/67, 2883/67, 8693/67, 8923/67, 9654/67 and 11344/67, and British Pat. Specification Nos. 950,443, 1,046,409 and 1,091,141. In these methods, the rate of supplying the monomers becomes low in forming the capsule film and the supplying thereof finally stops. Consequently, the resulting microcapsules generally have a thin capsule film which is a typical semipermeable membrane. In the "in situ" polymerization process, film forming materials are supplied to either the inside or the outside of the drops of the core material, and consequently polymerization occurs at the surface of the drops of the core material. Since most known polymerization reactions can be utilized, many kinds of capsule films can be formed.
A number of patents concern methods in which an oily monomer and core materials coexist, for example as described in Japanese Pat. Publication No. 9168/61, British Pat. Specification No. 1,237,498, French Pat. No. 2,060,818, and 2,090,862. Methods for producing a polymer film on the surface of core material by applying a film forming material from the dispersion medium are described in British Pat. Specification No. 989,264, Japanese Pat. Publication No. 14327/62 and 12380/62.
In the capsule films produced by these methods, film formation is not sufficiently carried out in general, and, consequently, the porosity of the capsule films is comparatively high.
Methods for microencapsulating utilizing a physical process, include a phase separation method using an aqueous solution and a drying method comprising drying in a liquid.
The phase separation methods using an aqueous solution comprise separating a thick polymer phase from an aqueous solution of a water soluble polymer. These methods have been practically utilized for many purposes at present. Such methods include a complex coacervation process and a simple coacervation process.
The method utilizing complex coacervation are described in U.S. Pat. Nos. 2,800,457, 3,116,206, 3,687,865, 3,265,630 (Japanese Pat. Publication No. 7726/62), 3,190,837 (Japanese Pat. Publication No. 7724/62), and 3,041,289. As methods for hardening capsule films formed, methods are described in Japanese Pat. Publication No. 3878/62, 3876/62, 3877/62, 12376/62, 24782/62, and U.S. Pat. No. 3,401,123, wherein formaldehyde, glutaraldehyde and glyoxal are used as a hardening agent.
The capsule films formed by these methods have substantially poor resistance to water or moisture and undergo swelling or permit permeation of the contents, because they are produced from water soluble polymer starting materials. Further, low molecular weight materials can pass through capsule films formed because the films per se are porous. Furthermore, the contents (the encapsulated materials) can be extracted by alcohols, ethers or ketone solvents. Methods utilizing simple coacervation are described in U.S. Pat. No. 2,800,458, French Pat. No. 1,304,891, Japanese Pat. Publication No. 7727/62, 7731/62 and 9681/62.
The capsule films formed by these methods have the same properties as the capsule films produced by complex coacervation.
The drying method comprising carrying out drying in a liquid comprises dispersing a solution of a capsule film forming material containing core materials in a encapsulating medium and volatilizing the solvent to form rigid capsule films.
This method has been described in Japanese Pat. Publication Nos. 13703/67, 28744/64 and 28745/64.
The capsule films formed by this method are usually a thin semipermeable membrane. Accordingly, they have the disadvantage that low molecular weight core materials penetrate through the capsule film.
Typical methods for producing capsules and the characteristics of the capsule films formed have been described above. But, additionally, phase separation methods using an organic solvent (e.g., the methods described in Japanese Pat. Publication No. 12379/62 and U.S. Pat. No. 3,173,878) and drying methods comprising drying in a liquid (e.g., the methods described in Japanese Pat. Specification No. 28744/64 and 28755/64) are known, but they are not satisfactory because of the thickness and density of the capsule films.
An object of the present invention is to eliminate the technical problems of the above described numerous encapsulation methods and to provide a method of forming capsule films having improved "protective ability for the encapsulated materials" which is an ideal characteristic for "microcapsules".
Herein, the term "improvement of protective ability" means that the density of the formed capsule film is increased, that the permeability to water and resistance to light is reduced, that the degree of swelling by water or moisture is decreased and that the strength is increased.